In the wet treating step of the semiconductor-manufacturing process, it is essential to dry the semiconductor wafer without causing any water marks to be formed on the wafer. In prior art drying techniques, there are known "dry spinning" methods wherein a wafer is rotated to dry by spinning out, and "IPA vapor drying" methods wherein a wafer is transferred to an atmosphere of an IPA vapor and dried. These techniques commonly have the problem that water marks are formed during the course of transfer from final rinsing to drying. Attention has now been made, as a substitute therefor, to techniques of the "Marangoni drying" method, wherein a wafer is dried by transferring from water to an IPA vapor atmosphere, and the "drying by direct substitution with IPA" method, wherein a layer of IPA is formed on the surface of the water to directly substitute the water with IPA.
In the conventional wet treatment methods, a problem arises due to the large size of the treating apparatus. In an apparatus provided with plural chemicals, treating vessels and rinsing vessels, the occupied area of the apparatus has to become large. In order to cope with this problem, it is effective to use an apparatus of the one-bath type wherein pure water and a plurality of solutions of chemicals are fed to one treating vessel.
An apparatus which has been intended to solve these two problems at the same time, is a one-bath type Marangoni dryer or a direct-substitution-with-IPA dryer. The outline of a typical apparatus is shown in FIG. 14. In FIG. 14, a wafer 2 is immersed in a treating vessel (treating chamber) 1 wherein the chemicals solution is upflown from a feed port 3 provided at a lower portion of the treating vessel 1 to etch the wafer 2. Thereafter, pure water is upflown from the feed port 3 for rinsing. The chemicals solution and pure water are overflown from the treating vessel 1 and discharged from a discharge port 4. During the drying of the wafer 2, IPA vapor is introduced from a feed port 5 into an upper space 1a of the treating vessel 1, and the wafer 2 is pulled up therein. This apparatus is of the type wherein the wafer 2 is elevated at the time of the drying.
FIG. 15 is a schematic view of another type of known wet treating apparatus. In this apparatus, a chemicals solution is passed for etching from a feed portion 3 provided at an upper side portion of a treating vessel 1. Subsequently, pure water is passed from the feed port 3 for rinsing. The chemicals solution and pure water are discharged from a discharge port 4 at a lower portion of the treating vessel 1. During the drying of the wafer 2, the level of the pure water in the treating vessel 1 is lowered and, at the same time, IPA vapor is introduced into an upper space of the treating vessel 1, followed by movement of the wafer 2 toward the IPA atmosphere. This apparatus is of the type wherein the water level is dropped at the time of the drying.
It has been found that where a silicon oxide film (SiO.sub.2) is etched with hydrofluoric acid (HF) by use of these apparatus, the silicon substrate is deposited with a multitude of silicon particles on the surfaces thereof.
Suppose a Si wafer and a silicon oxide film are simultaneously immersed in a PVC vessel. When the oxide film is etched with HF to a depth of 200 .ANG. and rinsed with water, and then subjected to the Marangoni drying, several thousands of particles are deposited on the Si wafer.
The formation mechanism of the particles are considered as follows.
Where the silicon oxide is etched with hydrofluoric acid, the following reaction (1) proceeds to the right side EQU SiO.sub.2 +6HF=H.sub.2 SiO.sub.6 +2H.sub.2 O (1)
When HF is present, the reaction proceeds to the right side. In a rinsing sequence, however, the reaction (1) proceeds to the left side. According to this reverse reaction, SiO.sub.2 is generated. The SiO.sub.2 is present as Si(OH).sub.4 colloids within the treating vessel, on the wall surfaces of the vessel, and on the surfaces of the wafer.
It has been found that during the "Marangoni drying" method or the "direct-substitution-with-IPA drying" method, IPA which is in the vicinity of the surfaces of the treating vessel in the drying sequence is dissolved and reacts with colloidal Si(OH).sub.4 thereby forming particles. Because of the formation of the particles, the one-bath type "Marangoni dryer" or the "direct-substitution-with-IPA type dryer" has placed limitations on applications thereof.
As stated hereinabove, with the known one-bath type apparatus of the wet treatment of semiconductor wafers, semiconductive particles are inevitably deposited on the semiconductor wafer after drying, thereby presenting a problem in subsequent semiconductor treating processes. If no particles are deposited, this method can be applied to the rinsing step of all the semiconductor wafer treating processes.